With the spread of the digital technology, the importance of a paper type display for displaying digital information (hereinafter referred to as “electronic paper”) has been increasing. The performance required for the electronic paper includes a high visual recognition and a low electric power consumption. High visual recognition means white background similar to paper, and hence a display method based on light-scattering white background similar to paper is suited. On the other hand, as to the electric power consumption, the reflection type display system has a lower power consumption as compared with that of self light-emission display system. Many systems have been proposed so far for the electronic paper. Examples include a reflection type liquid crystal display system, electrophoresis display system, magnetophoresis display system, dichroic ball rotation system, electrochromic display system, and leucothermal system. Any of these methods is not satisfactory from the viewpoint of high visual recognition, and improvement therefore has been demanded.
Many liquid crystal element (liquid crystal display element) systems have been proposed, and among the systems, the guest-host type liquid crystal element is capable of displaying a bright image, and is expected to be a liquid crystal element suited to a reflection type display system. In the guest-host type liquid crystal display, a liquid crystal composition containing a dichroic dye dissolved in a nematic liquid crystal is sealed in a cell, an electric field is applied thereto, and the orientation of the dichroic dye is changed depending on the movement of the liquid crystal by the electric field, and the light absorption state of the cell is changed to display an image. The dichroic dye has a uniaxial light absorbing axis, and absorbs only the light oscillating in the direction of the light absorbing axis, so that the light absorbing state of the cell can be changed by changing the orientation of the dichroic dye and controlling the direction of the light absorbing axis in accordance with the movement of the liquid crystal cell due to the electric field. This guest-host type liquid crystal cell is expected to be able to display a brighter image, as compared with the conventional liquid crystal cell, since a drive system without using a polarizer is possible. On the other hand, a polymer dispersion type liquid crystal display system, in which a polymer and a liquid crystal molecule are mixed has been proposed. In this system, since an eye-friendly display is possible because of light-scattering white background; orientation film and orientation process are not needed; manufacture by coating is possible, it is expected that an inexpensive display device can be obtained. In particular, a polymer dispersion type liquid crystal display system prepared by mixing a siloxane polymer and a fluorine-type nematic liquid crystal is proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-250939). This system is a light-scattering method in which the light-scattering and light-transmitting state is controlled. In this method, however, the scatter luminance is low and display performance is not satisfactory yet, and improvement has been demanded.
A guest-host polymer dispersion liquid crystal display system in which a polymer dispersion system and guest-host system are combined, has been proposed (see, for example, JP-A No. 9-40955). In this system, since a drive system without using polarizer is possible, an eye-friendly display based on a light-scattering white background is possible. As a low molecular-weight liquid crystal, a cyano-type liquid crystal E-7 is used. In the conventional system, however, the display performance is not satisfactory yet, and improvement has been demanded.
It has been known that the display contrast of the guest-host type liquid crystal display element varies with an order parameter of a dichroic dye in a liquid crystal composition for constituting a liquid crystal layer, or a host liquid cell or a cell structure. Generally, when the liquid crystal composition, in which a nematic liquid crystal and dichroic dye is combined, is disposed between substrates which is subjected to an orientation treatment, only the linearly polarized light at one side is absorbed, and the half of the light is transmitted, because the nematic liquid crystal assumes a uniaxial orientation state, so that the display contrast ratio cannot be enhanced. When a liquid crystal composition in which a nematic liquid crystal and a dichroic dye are injected between substrates which have not been subjected to a orientation treatment, the composition assumes an multi-domain state, and only the linearly polarized light of one side is absorbed and half of the light is transmitted, so that the display contrast ratio is not enhanced.
As methods for absorbing lights in all directions, a phase transition system, a ¼ wave plate system, and mode of dispersing a guest-host liquid crystal droplets in polymer (GH-PDLC) have been proposed.
In the phase transition system, due to the influence of the refractive index anisotropy of the liquid crystal to be used, the absorbance may be reduced by the rotation of the incident light along the spiral structure of the liquid crystal.
In the ¼ wave plate system, a metal reflector plate is needed to prevent the cancellation of polarized light, and a white background due to light-scattering cannot be obtained.
In contrast, in the GH-PDLC, axial directions become random among liquid crystal droplets, and lights in all directions can be absorbed. However, a high voltage is required to move the liquid crystal bound by a polymer network, and the display contrast ratio is not increased.
In order to solve these problems, a system using a liquid crystal polymer having liquid crystalline groups in the polymer side chain portion has been proposed. In this system, the side chain liquid crystal molecules respond to the application of voltage, and the display contrast ratio is enhanced. A polymer main chain having a small dielectric constant is preferable, and it has been known that a liquid crystalline siloxane polymer is preferable. (see, for example, JP-A No. 9-40955). In the mixed liquid crystal, the switching of the liquid crystal element is performed by changing the orientation of the liquid crystal by on/off switching of the electric field, and improvement of responsive speed has been demanded. On the other hand, it has been proposed “two-frequency driving method” in which the orientation of a liquid crystal is reversibly changed by applying an electric field by utilizing a two-frequency driving liquid crystal which changes the dielectric constant anisotropy Δ∈ from being positive to being negative with an increase in the frequency of voltage to be applied, (see, for example, Applied Physics Letters, Vol. 25, No. 4, 186-188, 1974). In this method, the element can actively be switched off, and by making use of this advantage of a fast responsive speed, and the method was once used in projector applications.
The GH-PDLC using this two-frequency driving liquid crystal has been known (see, for example, JP-A No. 9-33894). However, the polymer is acrylate-type oligomer which does not have liquid crystalline groups, and this system is not satisfactory in light of the display contrast and driving voltage. Accordingly, a liquid crystal element having a higher contrast ratio and a faster responsive speed is desirable.